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United States Patent |
5,165,409
|
Coan
|
*
November 24, 1992
|
Tonometry apparatus
Abstract
Apparatus for measuring pressure within an eye includes means for
increasingly inwardly deforming a surface of the eye by applying a
progressively increasing force onto the surface, means responsive to
inward deformation of the eye surface for taking a measure related to the
amount of the deformation and means for taking a measure related to the
amount of the applied force at a sampling time during the progressively
increasing force, and electronic means responsive to the
deformation-related measuring means and the applied force-related
measuring means for determining intraocular pressure from the relation
between the measures. Such apparatus includes a housing; a member
interposable between a surface of the eye and a portion of the housing,
and frontwardly-and-rearwardly moveable with respect thereto; means
associated with the housing and the member for providing a force, for
example a magnetic repulsion, resisting relative rearward movement of the
member so that when the member is interposed between the eye surface and
the housing portion and the housing portion is moved toward the eye
surface, the member front end deforms the eye surface and the member is
moved rearward with respect to the housing against the resisting force;
and means such as a Hall effect sensor for determining the position of the
member with respect to the housing portion. Also, such apparatus further
includes means associated with the member front end for determining the
amount of deformation of the eye surface.
Inventors:
|
Coan; William M. (39 Southfield Cir., Concord, MA 01742)
|
[*] Notice: |
The portion of the term of this patent subsequent to August 28, 2007
has been disclaimed. |
Appl. No.:
|
508762 |
Filed:
|
April 12, 1990 |
Current U.S. Class: |
600/405; 600/401 |
Intern'l Class: |
A61B 003/16 |
Field of Search: |
128/645-652,774
73/862.36,862.47,862.48,79
|
References Cited
U.S. Patent Documents
3308653 | Mar., 1967 | Roth | 128/645.
|
3934462 | Jan., 1976 | Rende | 128/652.
|
4729378 | Mar., 1988 | Trittenbass | 128/645.
|
4766904 | Aug., 1988 | Kozin et al. | 128/652.
|
4860755 | Aug., 1989 | Erath | 128/652.
|
Foreign Patent Documents |
3332724 | Mar., 1985 | DE | 128/645.
|
0457466 | Mar., 1975 | SU | 128/645.
|
1168187 | Jul., 1985 | SU | 128/645.
|
Primary Examiner: Aschenbrenner; Peter A.
Attorney, Agent or Firm: Choate, Hall & Stewart
Parent Case Text
BACKGROUND OF THE INVENTION
This application is a continuation-in-part of copending application U.S.
Ser. No. 235,347, filed Aug. 23, 1988, hereby incorporated by reference.
Claims
I claim:
1. Apparatus for measuring pressure within an eye, comprising
a housing,
a member interposable between a surface of the eye and a portion of said
housing, said member having a rear end and a front end and being
substantially non-compressible along its front-to-rear direction, said
member being engaged with said housing such that it is
frontwardly-and-rearwardly moveable with respect to said portion of said
housing,
means associated with said housing and said member for providing a force
resisting rearward movement of said member with respect to said portion of
said housing, so that when said member is interposed between the eye
surface and said housing portion and said housing portion is moved toward
the eye surface, said member front end forms a depression or flattening of
the eye surface and said member is moved rearward with respect to said
housing against said resisting force, and
means for determining the position of said member with respect to said
portion of said housing.
2. The apparatus of claim 1, further comprising means associated with said
member front end for determining the amount of depression or flattening of
the eye surface.
3. The apparatus of claim 1 or 2 wherein said means for providing said
force resisting rearward movement of said member comprises an elastic
membrane having a front surface against which said rear end of said member
presses as said member is moved rearwardly with respect to said housing
portion.
4. The apparatus of claim 3, said housing portion comprising an air chamber
and said elastic membrane comprising a front wall of said air chamber.
5. The apparatus of claim 4 wherein said means for determining said
position of said member with respect to said housing portion comprises a
pressure sensor responsive to pressure within said air chamber.
6. The apparatus of claim 4 wherein said air chamber comprises a bladder.
7. The apparatus of claim 4 wherein said pressure sensor comprises a
pressure transducer.
8. The apparatus of claim 4 wherein said elastic membrane is formed of
latex rubber.
9. The apparatus of claim 4, further comprising means for recording said
measure.
10. The apparatus of claim 4, further comprising a valve which when open
provides communication between said air chamber and atmospheric air.
11. The apparatus of claim 1 or 2, further comprising a prophylactic
membrane interposed between said front end of said member and said eye
surface.
12. The apparatus of claim 1 wherein said means for determining said
position of said member with respect to said housing comprises a Hall
effect sensor.
13. The apparatus of claim 1 or 2 wherein said means for providing said
force resisting rearward movement of said member comprises a magnet
associated with said member and a magnet associated with said housing, the
polarities of said member-associated magnet and said housing-associated
magnet being oriented such that increasing rearward movement of said
member with respect to said housing is increasingly resisted by magnetic
repulsion between said magnets.
14. Apparatus for measuring pressure within an eye, comprising
means for increasingly inwardly deforming a surface of the eye by applying
a progressively increasing force onto the surface,
means for taking a measure related to the amount of the deformation and
means for taking a measure related to the amount of said applied force at
a sampling time during said progressively increasing force, and
electronic means responsive to said deformation-related measuring means and
said applied force-related measuring means for determining intraocular
pressure from the relation between said deformation-related measure and
said force-related measure.
15. The apparatus of claim 14, further comprising means for automatically
selecting said sampling time.
16. The apparatus of claim 15, said automatically selecting means being
capable of selecting a plurality of said sampling times.
17. The apparatus of claim 15, said selected sampling time being a time at
which a predetermined force-related measure has been reached.
18. The apparatus of claim 15, said selected sampling time being a time at
which a predetermined deformation-related measure has been reached.
19. The apparatus of claim 15, said selected sampling time being the time
at the end of a predetermined time interval.
20. The apparatus of claim 14 wherein
said inwardly deforming means comprises a first member having a front end
and said progressively increasing force is applied onto said eye surface
by directing said front end toward the eye surface and moving said first
member toward the eye, and wherein
said means responsive to inward deformation of the eye surface comprises a
second member attached to said first member in frontwardly-to-rearwardly
moveable relation, said second member being moved increasingly rearwardly
with respect to said first member in response to increasing deformation of
the eye surface.
21. The apparatus of claim 20 wherein said first member comprises a shaft
having a tip that abuts a center of deformation of the eye surface during
said inward deformation and said second member comprises a generally
cylindrical collar mounted in sliding relation upon said shaft, said
collar having a front portion that abuts portions of an annular region of
the eye surface radially apart from the center of deformation.
22. A method for measuring intraocular pressure in an eye, comprising
providing means for inwardly deforming a surface of the eye,
contacting said deforming means with a surface of the eye and applying a
progressively increasing force to urge said deforming means in a direction
toward the eye to increasingly inwardly deform the surface,
measuring the amount of said deformation of the surface and the amount of
said applied force at a time during said progressively increasing force,
and
determining intraocular pressure from the relation between said measured
deformation and said progressively increasing force.
23. The method of claim 22 wherein said time is the time at which a
predetermined force is applied.
24. The method of claim 22 wherein said time is the time at which a
predetermined deformation has occurred.
25. The method of claim 22, 23 or 24, further comprising measuring the
amount of said deformation of the surface and the amount of said applied
force at a plurality of times during said increasing force.
Description
This invention relates to measuring intraocular pressure.
Nearly one percent of the total population of the United States suffers
from a form of blindness known as Glaucoma. Glaucoma is characterized by
an increase in pressure within the eye, which causes visual defects and
ultimately may cause irreversible blindness. As the intraocular pressure
rises to abnormal levels, damage is caused to the ocular nerve and
surrounding retinal tissues. The patient seldom experiences any symptoms
that might indicate that the disease exists until major damage occurs.
Typically, the patient's intraocular pressure is elevated, the retinal
field is seriously diminished, ocular nerve damage can occur, and there
may be some degree of pain.
As part of many standard eye examinations, a test of intraocular pressure
(tonometry) is performed to detect the early stages of glaucoma.
A measure of the pressure within the eye is conventionally obtained by
depressing to a given depth or flattening to a given extent a portion of a
measurement surface of the eye, usually the cornea, and then determining
the amount of force required to produce the given flattening or
depression. The flattening or depressing is resisted by the resiliency of
the measurement surface and by the internal pressure of the eyeball. The
determined force is then converted to a measurement of intraocular
pressure.
Commonly, the flattening or depression is produced by contacting the tip of
an instrument directly onto the measurement surface, and then pressing the
tip, whose dimensions are known, against the surface toward the eye. For
such measurements to be accurate, the tip of the instrument must be
properly oriented with respect to the measurement surface and the
direction of pressing toward the eye must be substantially normal to the
measurement surface.
Measurement of intraocular pressure using conventional apparatus generally
requires that the operator subjectively judge the depth, approach angle,
and position of the measuring instrument upon the eye. Operation of such
devices depends upon operator skill and consistency. Operator error and
the combination of these subjective variables can result in variability in
measurements taken by a particular operator from time to time, as well as
inconsistency in measurements taken by different operators.
Many tonometers in use employ optical systems that allow the clinician to
monitor the amount of flattening or depression of the eye surface to
adjust the proper angle and depth of penetration. Such devices typically
include a lens that rests directly upon the eye, through which the
operator views the tear meniscus to judge the correctness of the
flattening or depression at the time of measurement. Such devices
typically require equipment for holding the patient's head in a particular
position for a time, and often employ slit lamp equipment to aid in
alignment. Some time is required to set up such apparatus preparatory to
each measurement.
It is generally accepted that, when properly and skillfully used, direct
contact tonometers can give more valid indications of intraocular
pressure, and can provide more reliable diagnosis of early stages of
glaucoma, than can other types of tonometers.
The anterior chamber within the eyeball behind the cornea and in front of
the lens is filled with a fluid termed the aqueous humor. In the healthy
eye, the aqueous humor circulates out from the anterior chamber, and
glaucoma can be caused by a partial or full obstruction restricting this
flow. A consequent rise in intraocular pressure can occur long after the
restriction of flow has begun, so that the disease process may have been
under way for some time before any elevated intraocular pressure can be
detected.
In the healthy eye the aqueous humor can be caused to flow out from the
anterior chamber more rapidly by pressing inward onto the cornea for a
time, in effect massaging the aqueous humor out from the anterior chamber,
resulting in a measurable reduction of pressure within the eye. In a
technique known as tonography, the capacity for flow of the aqueous humor
from the anterior chamber is determined by imposing pressure onto the
cornea, typically by resting a weight upon the corneal surface while the
patient is in a supine posture and then taking a time series of
intraocular pressure measurements. Tonography is being used with
increasing frequency, as tonography has been found to yield greater
diagnostic and prognostic value that conventional tonometry alone for some
pathologies.
A variety of disease pathogens can be found on the surface of the eye, and
particularly in the fluid film that covers the eye. These include, for
example, pathogens causing herpes and, possibly, acquired immune
deficiency syndrome (AIDS). A disadvantage of conventional direct contact
tonometers is that because they must touch the eye, they can transmit such
diseases from eye to eye and from patient to patient.
It has been suggested that direct contact tonometers be provided with
disposable prophylactic covers, for preventing transmission of disease
pathogens from one eye to another. Damage to the eye sometimes occurs,
owing to individual tissue susceptibility to injury, to mishap, or to
operator error. Many known tonometers, including those which the operator
aligns by employing a lens in contact with the measurement surface, cannot
be modified to accommodate such covers. Those devices that have been so
modified are, at least partly as a result, not sufficiently sensitive to
provide accurate measurements, and they have not been accepted by the
medical community as clinically practical measurement devices.
Apart from error due to subjective judgments and error of the operator,
measurement error often is an effect of the design of the particular
device, and especially of the particular transduction scheme.
In early tonometers, lever systems were actuated by metallic springs or
cams to provide a mechanical analog of the intraocular pressure. The
precision of such devices depends upon the characteristics of
compressibility of the spring systems and individual units produce
differing measurements to the extent that their spring systems differ.
Spring fatigue and changes in temperature can cause changes in
measurement.
In other known devices strain gauges are directly coupled to the
measurement surface through a metal shaft that directly contacts the eye.
These, too, are affected by variations in temperature, and they can be
plagued with signal conditioning problems and poor schemes for calibrating
the strain gauge bridges. Some such systems cannot be calibrated by the
user and consistency in manufacture or materials cannot be assured in the
commercial production of such instruments.
In other known tonometers a piezoelectric crystal transducer is directly
coupled to a metallic plunger which directly contacts the eye.
Piezoelectric crystals can be affected by small and practically
undetectable changes in temperature. Because piezoelectric transducers
respond not only to force but also to temperature, varying temperatures in
ambient air as well as body heat transferred through the shaft from the
patient to the piezoelectric crystal can interfere with precise
measurement of the intraocular pressure. Moreover, the piezoelectric
crystal transducer can be affected not only by temperature but also by the
velocity with which the force applied to the measurement surface changes.
Such devices can yield a voltage analog signal that combines contributions
of the applied force, the velocity at which the force is increased, and
the temperature of the test environment. Further, such devices typically
require use of a microprocessor for calibration and for adjustment for the
non-linearity of the transducer mechanism. The user cannot easily
recalibrate the device in the field.
In devices known as "non-contact" or "airpuff" tonometers, compressed gases
are directed at the cornea to flatten or depress it. These are referred to
as "non-contact" devices because apart from one or more bursts of air
fired or released toward the eye from a predetermined distance they do not
come into contact with the measurement surface. Typically in such devices
an incident light wave is transmitted by light emitting diodes to the
cornea, which reflects it back to phototransducers within the device. As
the measured pressure of the compressed gas jet is directed toward the
measurement area, the surface is flattened and a measure of reflected
light yields a relative measurement of intraocular pressure. Such devices
are generally considered cumbersome to operate, and obtaining consistent
measurements depends upon the skill and technique of the operator in
aligning the instrument at the proper distance and orientation to the eye.
They are generally regarded by health care workers as useful initial
screening devices, but they are not generally accepted as providing
accurate measurements. The major attribute of their acceptance has been
the isolation of the patient from the device to inhibit disease
transmission. The patient often complains of the pain associated with the
blast of air that must be delivered to the eye to obtain a measurement,
and many patients are often reluctant to be measured a second time by such
devices.
SUMMARY OF THE INVENTION
Tonometry apparatus according to the invention employs means for applying
force upon a surface of the eyeball to deform, that is, to flatten or
depress, the eyeball surface, and means for measuring the amount of
deformation and for measuring the amount of force applied to the eyeball
surface, as described generally in my copending application U.S. Ser. No.
235,347. The intraocular pressure is determined by determining the amount
of deformation produced by applying a given force or, alternatively, by
determining the force required to depress or flatten the surface by a
given amount.
In general, in one aspect, the invention features apparatus for measuring
pressure within an eye, including an air chamber supported by a housing,
the air chamber having a deformable wall portion, a pressure sensor for
measuring air pressure within the chamber, and a member interposable
between a surface of the eye and the deformable wall portion. The member
has a rear end and a front end, is substantially non-compressible along
its front-to-rear direction, and is engaged with the housing such that it
is frontwardly-and-rearwardly moveable with respect to the deformable
chamber wall portion. When the member is interposed between the eye
surface and the deformable chamber wall portion, and the housing is moved
in a direction that shortens the distance between the eye and the air
chamber, the front end of the member inwardly deforms the eye surface and
the rear end of the member inwardly deforms the deformable wall portion,
raising the pressure within the air chamber, as measured by the pressure
sensor.
In preferred embodiments the air chamber is a bladder, preferably made of
latex rubber; the deformable portion of the air chamber wall is a
diaphragm, preferably an elastic diaphragm, most preferably made of an
elastomer such as latex rubber; the member includes a shaft coupled in
slidable relation to the housing; the pressure sensor includes a pressure
transducer; the apparatus further includes means responsive to the
pressure sensor for displaying a measure of the pressure; the apparatus
further includes means responsive to the pressure sensor for recording the
measure; the apparatus further includes a valve which when open provides
communication between the air chamber and atmospheric air; the apparatus
further includes means for aligning the member with the eye surface during
the measuring; the apparatus further includes a prophylactic membrane
interposed between the front end of the member and the eye surface.
In embodiments using a measure of the pressure in an air chamber as a
measure of the force used for flattening or inwardly deforming the eye
surface, the apparatus preferably employs a gas pressure coupling which
exerts equal pressure upon all legs of the transducer equally. The device
is not affected substantially by changes in ambient temperature, or by the
body temperature of the patient. Pressure equalization permits
measurements to be made independently of atmospheric pressure.
In general, in another aspect, the invention features apparatus for
measuring pressure within an eye, including a member, substantially
non-compressible along its front-to-rear direction and engaged with a
housing such that it is frontwardly-and-rearwardly moveable with respect
to the housing, means for resisting rearward movement of the member with
respect to the housing, and means for detecting the frontward-to-rearward
position of the member with respect to the housing. When the front end of
the member is interposed between the eye surface and the housing is moved
toward the eye, the front end of the member increasingly inwardly deforms
the eye surface while the rearward movement of the member with respect to
the housing is increasingly resisted, and the position of the member with
respect to the housing at the point where the eye has been deformed to a
specified degree provides a measure of the pressure within the eye.
I have now discovered that substantial improvements in performance of the
apparatus and in convenience of use can be obtained by moveably affixing
near the front end of the member a collar that is
frontwardly-to-rearwardly moveable with respect to the front end of the
member in response to deformation of the eye surface, and using a Hall
effect sensor and associated magnet in the probe tip for measuring the
front-to-rear position of the collar to provide a determination of the
amount of depression or flattening of the eyeball surface; or by employing
means for increasingly resisting the rearward movement of the member as
the member is forced rearward with respect to the housing, and using a
Hall effect sensor to determine the front-to-rear position of the member
in the housing as a measure of the rearward force on the member; or by
using Hall effect sensors in performing both measurements.
In preferred embodiments, the means for resisting rearward movement of the
member comprises a deformable wall portion of a closeable air chamber; the
apparatus further includes means for resisting frontward movement of the
member with respect to the housing; the means for resisting frontward or
rearward movement of the member with respect to the housing includes
magnetic field-producing means associated with the member and with the
housing; the magnetic field-producing means associated with the member
includes a magnet affixed to the member, preferably to the rear end of the
member, and the magnetic field-producing means associated with the housing
includes a magnet affixed to the housing; the magnets are arranged so that
rearward movement of the member with respect to the housing brings like
poles of a member-associated magnet and of a first housing-associated
magnet closer together; and so that frontward movement of the member with
respect to the housing brings like poles of a member-associated magnet and
of a second housing-associated magnet closer together; the polarities of
the magnetic fields are oriented along the direction of
frontward-and-rearward movement of the member; the magnets are axially
aligned with respect to the frontward-and-rearward movement of the member;
the position detecting means includes a Hall effect sensor, preferably
affixed to the housing and responsive to position of a magnetic field
associated with the member or affixed to the member and responsive to
position of a magnetic field associated with the housing; and the member
includes a rod having a rectangular, preferably square, section.
In general, in another aspect, the invention features apparatus for
measuring the pressure within an eye, including a member having a front
end arranged and adapted to deform the eye surface when the member is
pressed inward upon the eye surface, a force sensor arranged and adapted
to measure the force required for the front end to deform the eye surface
to a specified degree, and a deformation sensor for determining when the
eye surface has been deformed to the specified degree.
In preferred embodiments the deformation sensor includes a collar,
frontwardly-and-rearwardly moveable with respect to the front end of the
member in response to inward deformation of the eye surface by the front
end of the member, and a position sensor for determining the
frontward-to-rearward position of the collar with respect to the member;
preferably, the position sensor includes a Hall effect sensor affixed to
the member and arranged to be responsive to the position of a magnetic
field associated with the collar, more preferably to a magnetic field
produced by a magnet affixed to the collar.
In other preferred embodiments the deformation sensor includes an alignment
sensor including a plurality of contacts affixed to the member and an
element adapted and arranged to form electrical contact with one of the
contacts when the member is aligned with the eye surface and the eye
surface has been deformed to the specified degree. Preferably on these
embodiments the contact forming element is adapted and arranged to form
electrical contact with two of the contacts when the member is aligned
with the eye surface and the eye surface has been deformed to the
specified degree; the sensor includes at least three such contacts and the
element is adapted and arranged to form electrical contact with three of
the contacts when the member is aligned with the eye surface and the eye
surface has been deformed to the specified degree; the element includes an
element moveably affixed to the member, the element resiliently moveable
forwardly and rearwardly in relation to the member; the element includes
an annular piece; the contacts are sector shaped and each of the contacts
has a contact surface arranged in a plane perpendicular to a
frontward-rearward axis of the member; the apparatus further includes
means for resiliently urging the element frontwardly; the urging means
includes a spring; the spring includes a helical compression spring; the
urging means includes an elastic membrane.
The tonometer of the invention can be used in any of its embodiments to
accurately measure intraocular pressure through a disposable prophylactic
sheath, or prophylactic membrane.
The prophylactic sheath can be disposed of after each measurement and
replaced before the next measurement, thus helping to prevent transmission
from eye to eye and from patient to patient of pathogens that may be
present on the surface of the eye. Moreover, the prophylactic sheath helps
to ensure that the soft tissues of the cornea or the sclera, which are
contacted during measurement, are not scratched or otherwise damaged by
the instrument. Use of the prophylactic sheath does not interfere with
operation of the device or with precision or accuracy of measurement.
The sheath is smooth and non-irritating, and it helps to protect the eye
from direct injury that might result from operator error or inadvertent
movement of the eye during contact.
The precision of measurement is not affected by temperature deviations or
by changes in atmospheric pressure.
The apparatus can be used by persons having no special training or
experience, and provides accurate reproducible measurements objectively
without respect to the level of skill of the operator.
In some embodiments the apparatus coordinates an alignment sensing system
with a pressure measurement system so that a pressure measurement is held
and displayed automatically at the moment that specified alignment and
deformation criteria are met. The apparatus then notifies the operator
that the alignment criteria have been met and that a measurement has been
held and displayed.
In embodiments employing a deformation sensor in the form of a collar, and
using a Hall effect sensor to determine the rearward displacement of the
collar with respect to the tip, the tip assembly can be sufficiently
miniaturized that misalignment is unlikely to occur to an extent that
would result in inaccurate measurement, even in the hand of a relatively
inexperienced user; and a misalignment that exceeds a predetermined
tolerance can be automatically detected by the tip assembly, providing a
warning to the operator or preventing a record of a measurement.
In embodiments employing magnetic fields for resisting the frontward
movement of the member as well as its rearward movement, the
frontward-to-rearward position of the member is, in the absence of a
rearward force, effectively fixed by the competitive effects of the two
fields, irrespective of the orientation of the apparatus.
The tonometer of the invention can be employed with the patient in either a
supine or sitting position. Thus the invention provides for measurement of
intraocular pressure during ophthalmologic surgery, with the patient in
any of a variety of positions, or for ocular examination of patients who
cannot be elevated, as well as during customary routine examinations, with
the patients seated.
The device can be battery powered for improved portability and convenience
in use, and to avoid danger of electric shock to the patient. It can be
used with extremely low power digital and analog technology to preserve
battery power.
The apparatus can conveniently be used in tonography, by adding a weight to
the member, and allowing the front end of the member, carrying the weight,
to rest for a time upon the cornea of the supine patient, and performing a
time series of intraocular pressure measurements with the patient in a
supine posture.
DESCRIPTION OF PREFERRED EMBODIMENTS
Preferred embodiments of the invention will now be described, beginning
with a brief description of the drawings.
FIG. 1 is a perspective view of Tonometer apparatus of the invention.
FIG. 2 is a somewhat diagrammatic view of the apparatus of FIG. 1, partly
in section thru the axis of the force-measuring assembly, showing the
apparatus ready for taking a measurement.
FIG. 3 is a view of the apparatus as in FIG. 2, showing in sectional view
the force-measuring assembly in use during a measurement.
FIG. 4.1 is a somewhat diagrammatic view of a part of the apparatus of FIG.
1, made in section thru the axis of the probe tip assembly, showing the
probe tip assembly ready for taking a measurement.
FIG. 4.2 is a somewhat diagrammatic sectional view, taken at IV--IV in FIG.
4.1, thru the apparatus of FIG. 1.
FIG. 5.1 is a somewhat diagrammatic sectional view, as in FIG. 4.1, of the
apparatus of FIG. 1, showing the probe tip assembly in use in contact with
the eyeball surface during measurement.
FIG. 5.2 is a somewhat diagrammatic sectional view, taken at V--V in FIG.
5.1, thru the apparatus of FIG. 1.
FIG. 6 is a somewhat diagrammatic sectional view, taken as in FIG. 2, of
alternative force-measuring assembly according to the invention, showing
the apparatus ready for taking a measurement.
FIG. 7 is a somewhat diagrammatic sectional view of apparatus as in FIG. 6,
showing the apparatus in use during a measurement.
FIG. 8 is a somewhat diagrammatic sectional view, taken as in FIG. 2, of
alternative apparatus according to the invention, showing the apparatus
ready for taking a measurement.
FIG. 9 is a somewhat diagrammatic sectional view of the apparatus as in
FIG. 8, showing the apparatus in use during a measurement.
FIGS. 10 and 10.1 are diagrammatic views in perspective of a portion of the
force-measuring assembly of the apparatus in an embodiment employing
opposing magnetic fields to resist rearward movement of the member, as for
example in FIG. 2 or FIG. 8, illustrating arrangement and relative
movement of magnets on the moveable member and on the housing.
FIG. 11 is a sectional view thru the front-to-rear axis of the apparatus of
FIG. 10.
FIGS. 12 and 12.1 are diagrammatic views in perspective, as in FIGS. 10,
10.1, of alternative force-measuring assembly employing an additional
annular magnet, illustrating arrangement and relative movement of magnets
on the moveable member and on the housing.
FIG. 13 is a sectional view thru the front-to-rear axis of the apparatus of
FIG. 12.
FIGS. 14 and 15 are block diagrams, respectively, of alternative electrical
circuitry of the apparatus according to the invention.
STRUCTURE AND OPERATION
The invention provides tonometry apparatus in various embodiments capable
of providing accurate and reproducible measurements of intraocular
pressure, while permitting the use of a disposable sheath for covering the
part of the apparatus that touches the eye during measurement. In various
embodiments, the invention features probe tip apparatus which provides for
sensitive indication of proper depression or flattening of the eye
surface; and features apparatus which provides for accurate measurement of
the force necessary to produce the depression, unaffected by atmospheric
conditions in the testing milieu or by whether the patient is in an
upright or a supine posture, as a measure of the intraocular pressure.
Preferred tonometer apparatus of the invention is shown by way of example
in a perspective view in FIG. 1. Referring now to FIG. 1, a housing, shown
generally at 41, includes handle portion 1, from which nose portion 43
generally projects. A probe tip assembly, shown generally at 40, is
affixed to a shaft member 9, which is slidably engaged within housing 41
such that the shaft member and the probe tip attached to the shaft member
can slide frontwardly and rearwardly with respect to the housing, as
described more fully below. A display, shown generally at 300, is
connected to housing 1 via cable 42. Cable 42 contains wires (not shown in
FIG. 1) which provide for electrical connections between parts of display
300 and parts of housing 41, as further described below with reference to
drawings in sectinal view. Reset switch 32 is provided in housing 1 for
resetting the electrical circuits of the apparatus between readings, as
described below.
Referring now to FIGS. 2 and 3, shaft member 9 preferably is a rigid tube
having a rectangular (preferably square) section, and bushing 10 in
housing 41 provides a low-friction bearing surface for frontward and
rearward sliding movement of the shaft member. Probe tip assembly 40, as
described more fully below with reference to FIGS. 4.1 and 5.1, includes a
generally disc-shaped probe tip 13, affixed at the front end of shaft
member 9, and a generally cylindrical or collar-shaped probe tip housing
12, affixed to a bushing 11 which is slidably engaged with shaft member 9
to provide low-friction frontward and rearward movement of the probe tip
housing upon the front portion of shaft member 9. The probe tip housing
and the probe tip are covered by a removable and disposable prophylactic
latex cover 14 prior to use.
FORCE
With reference now particularly to FIGS. 2, 3, 10, 10.1 and 11, toroidal
magnets 30, 31 are affixed respectively to the rear end of shaft member 9
and to housing 1. Magnets 30, 31 are axially aligned so that their
polarities are antiparallel. For instance, the North magnetic pole of the
shaft magnet 30 might face rearward toward the North magnetic field of the
stationary magnet 31 or, as illustrated for example in FIGS. 10, 10.1 and
11, the South magnetic poles of the two magnets 31 and 30 could similarly
be arranged to face each other to yield the same effect.
As a result of this arrangement, the opposing magnetic fields generated by
magnets 30, 31 mutually repel when the magnets are approximated, producing
a force resisting rearward movement of the shaft member 9 with respect to
the housing 1. This resisting force increases as the shaft member is moved
further rearward, so that when the shaft is at rest the amount of rearward
force on the shaft can be determined by the front-to-rear position of the
shaft relative to the housing. The front-to-rear position of the shaft is
determined by shaft position sensor 33, which is mounted on housing 41.
Shaft position sensor 33 is a "Hall effect sensor", preferably of the
linear HE type, such as is available, for example, as Sprague UGN-3503U
(Concord, N.H.), and is arranged so that it is responsive to the magnetic
field produced by magnet 30. Thus, shaft position sensor 33 determines the
proximity of the shaft magnet 30 by producing an electrical current (or,
alternatively, raising an electrical potential) that is related to the
strength of the magnetic field produced by shaft magnet 30 and the
proximity of the shaft magnet 30 to shaft position sensor 33.
The magnetic sensor 33 can be arranged as illustrated for example in FIGS.
2, 3, 10, 10.1 and 11 such that when the shaft member 9 is moved rearward,
moving the magnet 30 away from the sensor 33 in a rearward motion, the
electrical current (or potential) from the sensor is reduced in proportion
to the strength of the magnetic field at the sensor 33 as the magnet 30
travels away from the sensor. In other words, in this arrangement, when
the magnet 30 is close to the sensor 33, the sensor produces a maximum
amount of current (or raises a maximum potential). When the magnet 30 is
driven rearward away from the magnetic sensor 33, the magnetic field
sensed by the sensor 33 is reduced and, consequently, the sensor produces
less electrical current (or raises a lower potential). As strength of the
magnetic field produced by the magnet 30 does not change, and as the
sensor 33 is responsive to the strength of the magnetic field at the
sensor, the sensor 33 detects different magnetic field strenghts as the
magnet is moved nearer or farther from the sensor. Thus, the magnetic
sensor provides an accurate indication of the position of the magnet 30
relative to the stationary magnetic sensor 33, which is calibrated to a
measure of the force applied to the eye by the tip.
When a rearward force is applied to the shaft member, causing the probe tip
13 to move rearward with respect to the housing 41 and the magnet 31,
shaft 9 brings the rear surface of magnet 30 toward magnet 31 and away
from magnetic sensor 33, as shown for example in FIGS. 3, 10, 10.1 and 11.
As the distance is increased between magnet 30 and magnetic sensor 33, the
magnetic sensor produces an electrical current (or raises an electrical
potential) whose magnitude is related to the position of the magnetic
field of magnet 30. In this arrangement, the sensor current (or potential)
falls as the distance increases between the magnet and the sensor,
providing an electrical analog of the distance between the magnet and the
sensor, and thus of the front-to-rear position of the shaft 9 in relation
to the housing 1. Because the rearward movement of the magnet 30 is
opposed by magnet 31, an increase in force upon probe tip 13 is necessary
to increase the relative distance between sensor 33 and shaft magnet 30.
The opposing force of the magnets can be calibrated in relation to the
distance of the shaft magnet 30 from the sensor 33, and in this way the
front-to-rear position of the shaft magnet in relation to the sensor may
be converted into an analog of the force or pressure applied to the shaft
foot plate 13. By plotting the force required to displace the shaft 9
rearward from point of rest toward the stationary magnet 31 (to a point
where the greatest opposing magnetic force is encountered) as a function
of the distance traveled by the magnet 30 from the sensor 31, an accurate
analogue of force from distance or proximity can be obtained.
With reference now to FIGS. 12, 12.1 and 13, the front-to-back position
that the shaft assumes when no rearward pressure is imposed on the tip can
be fixed by mounting a third annular magnet 49 on the housing 41 in a
position frontward from the shaft magnet 33. The polarity of magnet 49 is
axially aligned with, and antiparallel to, the polarity of shaft magnet
33, and shaft 9 passes through the hole in toroidal magnet 49, as
indicated for example by the letters N and S next to the magnets in FIGS.
12, 12.1 and 13. The start position for the shaft between measurements is
held by the mutually repelling magnetic fields of magnets 33, 49, and 31,
effectively irrespective of the orientation of the device.
Alternatively, the rearward movement of the shaft member 9 can be resisted
by resilient mechanical means, such as, for example, by a spring or by an
elastic membrane against which the rear end of the shaft member 9 presses.
In such embodiments, the rearward position of the member can be detected by
magnetic field sensing means, generally as described above, for example by
mounting a small magnet on the shaft member and placing a Hall effect
sensor on the housing so that as the shaft member 9 moves frontward or
rearward, the distance between the small magnet and the sensor changes.
Or, alternately, in an embodiment using an elastic membrane to resist
rearward movement of the shaft member, the membrane can form a deformable
wall portion of a closeable chamber, and a measure of the pressure within
the chamber can form a analog of the front-to-rear position of the member
(and thus of the rearward force on the member, imposed by the resistance
of the eye surface to deformation), as described generally in my copending
application U.S. Ser. No. 235,347.
With reference now to FIGS. 6 and 7, showing such an alternative embodiment
of the force-measuring means, disc-shaped plunger 8 is affixed to the rear
end of shaft member 9. Housing handle portion 1 contains air chamber 2,
enclosed by chamber wall 44, a portion of which is constructed of an
elastic membrane material, such as a latex rubber sheet, forming a
deformable wall portion 3. When shaft member 9 is moved rearward with
respect to housing 41, sliding in low-friction bushing 19, shaft member 9
brings the rear surface of plunger 8 in contact with the front surface of
deformable front portion 3 of wall 44. Continued rearward movement of
shaft member 9 causes plunger 8 to rearwardly displace deformable front
wall portion 3, as shown in FIG. 7.
Handle 1 contains a pressure transducer, shown schematically generally at
4, which communicates with air chamber 2 by way of air chamber port 7.
Pressure transducer 4 is provided with differential intake port 5, which
communicates with ambient air, and air chamber 2 communicates with
pressure equalizer valve 6, by which air can be taken into or exhausted
from chamber 2. When pressure equalizer valve 6 is closed, as will be
appreciated by one skilled in the art of pressure measurement, pressure
transducer 4 in effect compares the pressure within air chamber 2 via air
chamber port 7 with ambient atmospheric pressure via differential intake
port 5. When pressure equalizer valve 6 is open to the atmosphere, the two
pressures are equal. Rearward displacement of deformable wall portion 3
decreases the volume of air chamber 2; when air chamber valve 6 is open,
such a volume decrease results in exhausting a portion of the air out from
air chamber 2 through air chamber valve 6. On the other hand, when
pressure equalizer valve 6 is closed, rearward displacement of deformable
wall portion 3 compresses the air contained within air chamber 2 and
increases the pressure sensed by the pressure transducer via air chamber
port 7. Because the difference between the pressure within closed air
chamber 2 and ambient air is related to the extent of deformation of
deformable wall portion 3, which in turn is related to the extent of
movement of the shaft member 9 against the resistance of the deformable
wall portion 3, the pressure sensed by pressure transducer 4 provides an
analog of the position of the shaft member in relation to the housing. As
described above, this provides an analog of the pressure within the eye
whose surface is being deformed by pressing the probe tip inward against
the eye surface.
Housing 41 is preferably molded of a sturdy plastic. Wall 44 of air chamber
2 can be rigid except for the deformable portion, which can be made of any
resiliently deformable material, but which is conveniently made from latex
rubber sheet; or, alternatively, air chamber 2 can be formed entirely as a
bladder made of latex.
Pressure transducer 4 is preferably of the gage pressure type, such as, for
example, one of the SCX Series transducers, available commercially from
SenSym, 1255 Reamwood Avenue, Sunnyvale, Calif. 94089.
DEFORMATION AND ALIGNMENT
A measure of the depression or flattening of the surface of the eyeball by
the probe tip is made by means of the probe tip assembly 40. With
particular reference now to FIGS. 4.1, 4.2 and 5.1, 5.2, a probe tip
button 13 is affixed to the front end of shaft member 9. Collar-shaped
probe tip housing 12, affixed to bushing 11, is slidably movable
frontwardly and rearwardly upon the front portion of shaft member 9. The
front end of probe tip housing 12 is provided with an annular flange 45.
Probe tip button 13 and flange 45 are covered by a removable and
disposable prophylactic latex cover 14. As is described more fully below,
when the probe tip assembly, covered by the prophylactic sheath, is
brought into contact with the eye surface, the front surface of the probe
tip button 13 is at first approximately in the same plane as the front
surface of the flange 45. Then as the operator moves the apparatus toward
the eye to press the probe tip with increasing force against the eye
surface, so that the probe tip button inwardly deforms the eye surface, an
annular region M of the eye surface about the margin of the probe tip
button presses rearward against the front surface of the annular flange,
causing the collar-shaped probe tip housing to move rearward on the shaft
member. As the deformation increases, the probe tip housing is moved
further rearward and, as a result, the front-to-rear position of the tip
assembly 40 in relation to the shaft 9 can provide an analog of the amount
of deformation of the eye surface.
Following a measurement, tip assembly 40 can be returned to a more
frontward position (preferably bringing the front surface of flange 45
into a coplanar relation with the front surface of tip button 13) by use
of urging means such as, for example, magnetic repulsion having a
magnitude great enough to overcome frictional resistance between the
bushing 11 and the shaft member 9, but not great enough to contribute to
error in the measurement, that is, to interfere substantially with the
rearward force on the annular flange 45 by the deforming eye surface
during measurement. With reference to FIGS. 4.1 and 5.1, such an urging
force can be provided as follows. A small magnet 46 is secured to the
lumenal surface of the wall of hollow shaft member 9, and another small
magnet 47 is secured to the tip assembly. Magnets 46, 47 have their
magnetic poles oriented antiparallel and front-to-back with respect to the
movement of the bushing 11 on the shaft 9, as suggested by the + signs
near the magnets in the Figs. The repulsion by the opposing magnetic
fields urges the tip assembly 40 frontwardly with respect to the shaft
member, irrespective of the orientation of the device, and making the
apparatus useful with the patient facing upward or horizontally, or any
direction. This frontward urging force additionally aids in reducing
front-to-back displacement of the tip assembly that a latex sheath might
cause.
The front-to-rear position of the probe tip assembly 40 in relation to the
shaft member 9, and thus to the probe tip button 13, preferably is
measured by magnetic field sensing means, much as described above with
reference for example to FIGS. 2 and 3 for measuring the front-to-rear
position of the shaft member in relation to the housing by magnetic field
sensing means.
One arrangement of magnetic field sensing means for determining the
relative front-to-rear position of the probe tip assembly is shown by way
of example in FIGS. 4.1 and 5.1. Tip magnet 15 is affixed to bushing 11 in
such a position that tip magnet 15 will come into closer proximity to tip
position sensor 16 when tip assembly 40 is forced rearward by applying
force upon the most forward portion of tip assembly 40, that is, upon at
least a portion of the front surface of flange 45.
Magnet 15 is affixed to tip bushing 11, and tip position sensor 16 is
affixed by way of bracket 17 to shaft member 9. Tip position sensor 16 is
a Hall effect sensor, and is arranged so that it is responsive to the
magnetic field generated by the tip magnet 15 as the tip assembly 40
slides frontwardly and rearwardly upon shaft member 9, as shown
particularly in FIGS. 4.1 and 5.1. That is, as the tip assembly 40 moves
with respect to the shaft, the distance between tip magnet 15 and tip
position sensor 16 changes, resulting in a change in an electric signal
produced by the position sensor 16. The position of the tip assembly 40
with respect to the probe tip 13 is thus accurately indicated by the
signal produced by the tip position Hall effect sensor.
FIG. 4.1 is a somewhat diagrammatic sectional view of the probe tip
assembly ready for taking a measurement, and prior to placement upon the
eye. The magnet 15 produces a magnetic field which is detected by the
magnetic sensor 16. Because the distance between magnet 15 and magnetic
sensor 16 is at this point maximal, the magnetic field detected by the
magnetic sensor is minimal and the sensor produces a very small electric
current (or raises a relatively small potential). FIG. 5.1 shows a
rearward displacement of the tip assembly 40 as the shaft 9 is forced
against the surface of the eye. As the operator, holding the apparatus by
the handle portion of the housing, moves the apparatus toward the eye,
probe tip 13, overlain by a central portion of sheath 14, begins to
flatten and then to depress the measurement surface, as for example the
cornea. This deformation is resisted by the cornea and by the internal
pressure of the eye, and shaft 9 begins to move rearward with respect to
housing 41, forcing shaft magnet 30 and its accompanying magnetic field
toward the opposing stationary magnet 31 as described above generally with
respect to FIG. 3. Referring now to FIG. 5.1, as the deformation
increases, a depression forms in cornea C at the location P where probe
tip 13, covered by a central portion of the sheath 14, presses inward.
Concurrently, an edge of the depression forms at regions M of cornea C
situated radially away from location P. These regions press rearward
against the areas of sheath 14 overlying the edge of annular flange 45,
and this rearward pressure causes a rearward displacement of annular tip
assembly 40, bushing 11 and magnet 15, with respect to probe tip body 13
and shaft 9. As magnet 15 is displaced rearwardly, the magnetic field of
the magnet 15 comes into closer proximity to the sensor 16, which is
affixed to shaft 9 by bracket 17. As the magnet 15 approaches the sensor
16, the sensor 16 is exposed to the strengthening field of the magnet 15
and responds by delivering a ratiometric electric current (or potential)
in relation to the proximity and strength of the magnet. In other words,
the closer the magnet comes to the sensor, the stronger the magnetic field
becomes around the magnetic sensor and, consequently, the greater the
amount of current which the sensor produces. As the magnet 15 approaches
rearwardly toward the magnetic sensor 16, the magnetic field increases as
the distance between magnet 15 and magnetic sensor 16 decreases. Because
the magnetic sensor 16 is affixed to the shaft 9 via bracket 17, the
sensor 16 interprets the strengthening magnetic field as an indication of
the positional relationship of the housing 12 to the probe tip 13,
indicating the depth of depression of probe tip 13 into the cornea of the
eye whose intraocular pressure is being measured.
It will be appreciated that if the apparatus is misaligned as the operator
brings it toward the eye surface, that is, if it is not oriented with the
front-to-rear axis approximately normal to the eye surface, the eye
surface will contact a front part of the probe tip assembly and deform it
rearward with respect to the probe tip before the probe tip itself
contacts the eye surface. The greater the misalignment, the greater the
extent of such early rearward movement of the probe tip assembly, and if
the tip position sensor detects a substantial amount (that is, greater
than a predetermined threshhold amount) of rearward movement of the probe
tip assembly before the shaft position sensor detects any rearward
movement of the shaft member, then a misalignment condition is indicated,
and the apparatus can automatically so inform the user by a visual or
audial signal, or can prevent a measurement being recorded. The operator
can then withdraw the apparatus, reapproach the eye surface with improved
alignment, and attempt the measurement again. The threshhold can be
selected according to the alignment needs presented by the particular
measurement being sought.
An alternative arrangement of magnetic field sensing means for determining
the relative front-to-rear position of the probe tip assembly is shown in
FIGS. 8 and 9. In this example, as in the example described above with
reference to FIGS. 4.1 and 5.1, tip magnet 15 is affixed by bracket 17 to
bushing 11 in such a position that tip magnet 15 will come into closer
proximity to tip position sensor 16 when tip assembly 40 is forced
rearward by applying force upon the most forward portion of tip assembly
40, that is, upon at least a portion of the front surface of flange 45. In
this example, however, tip position sensor 16 is affixed to the housing 41
rather than to the shaft member 9. In this configuration, as the operator
moves the housing toward the eye, deforming the eye surface, the movement
of the magnet 15 rearward with respect to the tip position sensor 16 is a
sum of the rearward movements of the tip assembly with respect to the
shaft member and of the shaft member with respect to the housing. The
microprocessor, to which the sensors send their signals, as described more
fully below with reference to FIGS. 14 and 15, examines the output of the
sensors, and determines the component relative positions by, for example,
subtraction of the relative positional data provided by the shaft position
sensor 33 from the relative positional data provided by the tip position
sensor 15.
In alternative embodiments of the invention, means other than Hall effect
sensors can be used either in determining the degree of deformation or in
determining the force required to produce the deformation.
ELECTRONIC TREATMENT OF MEASUREMENTS
FIGS. 13 and 14 are block diagrams showing electronic circuitry for use in
conjunction with the probe apparatus of the invention. Electrical power
supplied by a DC power supply 101, such as a battery, passes to a voltage
and amperage 102 regulator to provide a predetermined reference potential
and current for the sensitive air pressure transducer bridge and amplifier
circuits 104, Hall effect circuits 103 and microprocessor controller
circuits 108.
The electronic circuitry includes components that produce pressure data
relating to the pressure within the air chamber (in embodiments in which
an increase in air chamber pressure caused by deformation of a deformable
wall surface of the air chamber by the rearwardly-moving shaft member
provides a measure of shaft position), components that are related to the
amount of corneal displacement, components that produce signals related to
the achievement of preprogrammed test paradigms 108, components related to
the display of the test and calibration data 110, 112, components related
to the timing of special test situations and components related to the
transfer of data to other monitoring devices 111 such as printers,
computers, meters and recorders, componenets related to reset circuits 107
for resetting or interrupting the microprocessor 108, and components
related to calibration of the Hall effect systems 105, 104 and of the
pressure sensor 113.
With reference to FIG. 14, as the cornea and the intraocular pressure force
the probe tip assembly and shaft rearward with respect to the housing,
pressing the plunger against the deformable wall of the air chamber and
increasing the air pressure within the chambers as described above with
reference to FIGS. 3 and 4, the pressure transducer produces an electrical
analog of the pressure developed within the air chamber. The electrical
analog is amplified by a pressure transducer ratiometric amplifier 113 and
referenced by a calibration circuit 114 and a signal conditioning circuit,
and the resulting potential is sent to an analog to digital (A/D)
converter, which can be contained within microprocessor controller 108,
which generates a digital representation of the pressure.
The digital pressure signal from the A/D converter can be either directed
to another microprocessor for additional treatment, or presented directly
to a display driver 110 and then to a display transducer 112, such as
liquid crystal display (LCD) which presents the intraocular pressure in
terms of millimeters of mercury (mm Hg). Any of various types of audio
signal can be produced by the audio signal generator 109, under control of
the microprocessor, for informing the operator of the working state of the
apparatus, such as, for example, informing that the apparatus is
improperly aligned, or a threshhold amount of force has been imposed on
the eye surface, or the eye surface has been deformed to a threshhold
degree, or a measurement has been made.
Referring now to FIG. 15, for use in embodiments where the rearward force
is measured using a Hall effect sensor, pressurer sensor amplifier and
calibration circuits 113, 114 are replaced respectively with Hall effect
amplifier and calibration circuits 104, 106, which perform generally
similar functions.
Because the apparatus according to the invention is capable of
simultaneously measuring and recording both the force applied to the eye
and the deformation of the eye as a function of the applied force, several
different test paradigms exist for achieving a measure of intraocular
pressure, for example as follows.
As the device is directed upon the eye, in the manner previously discussed,
the microprocessor can be software controlled or programmed to measure the
amount of corneal deformation as a function of the force or pressure
applied to the eye. In this testing configuration, the device is
programmed to measure the deformation of the test site at a predetermined
pressure or pressures such as 1, 2, 3, 4, 5, or more mm Hg or any
pressures in between these intervals. When a predetermined pressure is
reached, the microprocessor automatically measures the amount of
deformation of the measurement surface and stores the information for the
desired data treatment. Or, alternatively, as the device is directed upon
the eye, in the manner previously discussed, the microprocessor may be
software controlled or programmed to measure the amount of pressure or
force applied as a function of the deformation of the eye when increasing
force is applied. In this testing configuration, the device is programmed
to measure the pressure on the test site at one or more predetermined
deformation amounts. When a predetermined deformation is reached by
applying increasing force to the eye, the microprocessor automatically
measures or records the amount of pressure required to achieve the
appropriate amounts of deformation and stores the information for the
desired data treatment.
Or, alternatively, the nearly simultaneous recordings of both pressure and
deformation can be sampled at intervals over time by the microprocessor
and then stored and treated to achieve a measure of intraocular pressure.
USE
The tonometer apparatus of the invention can be used by an operator with
little training and having no special skills, generally as follows.
At the outset the operator fits a sterile sheath over the probe tip
assembly, following sterile procedure as will be familiar to health care
workers generally. Then the operator presses the reset button 32 and, in
embodiments in which the pressure within an air chamber is taken as an
analog of the front-to-rear position of the shaft member, pressure
equalization port 6 is opened to allow the air pressure within the air
chamber and the pressure transducer to equilibrate to atmospheric
pressure, which is sampled by the pressure transducer at differential
intake port 7. Once equilibration has occurred, pressure equalization port
6 is closed. At this point the pressure within air chamber 2 equals
ambient atmospheric pressure, unless or until the atmospheric pressure
changes or a force upon probe tip assembly 40 causes shaft 9 to move
rearward with respect to housing 1, causing plunger 8 to press against
deformable wall 3 of the air chamber.
Once equilibration has been completed, the operator brings the apparatus to
the eye, addressing the cornea with the probe tip assembly oriented as
nearly normal to the corneal surface as can be estimated. As the operator
presses the apparatus forward toward the eye, so that probe tip assembly
40 of the device is gently forced against the corneal surfaces, the
resistance exerted by the cornea and by the intraocular pressure causes
the shaft member to move rearward against the resistance means.
The probe tip assembly, employing a reawardly-moveable probe tip assembly
together with magnetic field position sensing means for determining the
extent of deformation of the eye surface according to the invention, can
be miniaturized sufficiently that the likelihood of misalignment, and the
magnitude of errors resulting from any misalignment that may occur, are
substantially reduced.
The operator repeats the attempt if necessary, retreating each time a
misalignment signal is heard until the alignment program informs the
operator, as described above with reference to FIGS. 14 and 15, that the
device has been pressed sufficiently far toward the eye that an depression
has been made, to the correct depth. The deformation measuring program
will at that moment have instructed the pressure measuring circuitry to
hold and display an analog of the intraocular pressure, and the operator
can take the pressure measurement from the display.
After the operator has examined the displayed measurement, the circuitry is
reset by means of a reset circuit button. The operator can then take
additional measurements on the same eye, or can replace the contaminated
sheath before making measurements on the next eye. The contaminated sheath
is replaced by removing it from the probe tip assembly, and the apparatus
is ready to be fitted with a sterile sheath and reequilibrated in
preparation for the next measurement.
OTHER EMBODIMENTS
Other embodiments are within the following claims. For example, one or more
of the various audible signals can be converted to visual display signals
to notify the operator that a measurement has been taken or that the
instrument is misaligned.
The probe tip can be pressed into the sclera rather than into the cornea.
Because the sclera itself resists deformation more firmly than the corneal
wall itself, a specified force against the sclera produces a shallower
depression than against the cornea of the same eye. The apparatus can be
calibrated to accommodate this difference.
The output of the A/D converter, which can be located within the
microprocessor, can be presented to other forms of microprocessors or
computers for additional analysis or to save test results in an electronic
memory or in a display or printout.
The device can be used without the sheath, but it is preferred that the
sheath be used, because it protects the corneal tissues from injury and
because it can be discarded between measurements, preventing the spread of
disease.
The device can also be used in the practice of tonography by affixing a
mass of desired weight (such as, for example, approximately 10 grams) to
the probe tip assembly such that it does not come into contact with the
eye and does not interfere with the front-to-back movement of the probe
tip assembly with respect to the probe tip on the shaft member. With the
patient placed in a supine position, the device is directed upon the eye
such that only the weight of the floating tip and its coupled 10 gram mass
may put pressure upon the measurement area of the cornea. At timed
intervals, the operator may apply a downward force upon the instrument,
forcing the probe tip against the eye surface and permitting taking a
measurement of the intraocular pressure at the point as described above.
After having taken the pressure measurement, the information is recorded
or saved by the device and the instrument is raised slightly from the eye
leaving only the full weight of the tip bearing and its coupled weight to
rest upon the eye until the next test interval. This process may be
repeated over two, four or more minute intervals, as is customary for
tonography.
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